Traditional printed circuits are often constructed in what is commonly called rigid or flexible formats. The rigid versions are used in nearly every electronic system, where the printed circuit board (PCB) is essentially a laminate of materials and circuits that when built is relatively stiff or rigid and cannot be bent significantly without damage.
Flexible circuits have become very popular in many applications where the ability to bend the circuit to connect one member of a system to another has some benefit. These flexible circuits are made in a very similar fashion as rigid PCB's, where layers of circuitry and dielectric materials are laminated. The main difference is the material set used for construction. Typical flexible circuits start with a polymer film that is clad, laminated, or deposited with copper. A photolithography image with the desired circuitry geometry is printed onto the copper, and the polymer film is etched to remove the unwanted copper. Flexible circuits are very commonly used in many electronic systems such as notebook computers, medical devices, displays, handheld devices, autos, aircraft and many others.
Flexible circuits are processed similar to that of rigid PCB's with a series of imaging, masking, drilling, via creation, plating, and trimming steps. The resulting circuit can be bent, without damaging the copper circuitry. Flexible circuits are solderable, and can have devices attached to provide some desired function. The materials used to make flexible circuits can be used in high frequency applications where the material set and design features can often provide better electrical performance than a comparable rigid circuit.
Flexible circuits are connected to electrical system in a variety of ways. In most cases, a portion of the circuitry is exposed to create a connection point. Once exposed, the circuitry can be connected to another circuit or component by soldering, conductive adhesive, thermo-sonic welding, pressure or a mechanical connector. In general, the terminals are located on an end of the flexible circuit, where edge traces are exposed or in some cases an area array of terminals are exposed. Often there is some sort of mechanical enhancement at or near the connection to prevent the joints from being disconnected during use or flexure.
In general, flexible circuits are expensive compared to some rigid PCB products. Flexible circuits also have some limitations regarding layer count or feature registration, and are therefore generally only used for small or elongated applications.
Rigid PCBs and package substrates experience challenges as the feature sizes and line spacing are reduced to achieve further miniaturization and increased circuit density. The use of laser ablation has become increasingly used to create the via structures for fine line or fine pitch structures. The use of lasers allows localized structure creation, where the processed circuits are plated together to create via connections from one layer to another. As density increases, however, laser processed via structures can experience significant taper, carbon contamination, layer-to-layer shorting during the plating process due to registration issues, and high resistance interconnections that may be prone to result in reliability issues. The challenge of making fine line PCBs often relates to the difficulty in creating very small or blind and buried vias.
The process used by current technology is based upon a dry film process, where a substrate of some sort has a copper layer as the base circuit layer onto which a dry film is applied. The dry film is then patterned with a laser to create the circuit patterns. The next copper layer is added and etched as appropriate, with the laser used to drill through the film to expose the previous copper layer so a via can be plated to join the circuit layers. This process is typically used for semiconductor package substrates and larger format circuit boards, such as used in a cell phone. For larger format circuit boards, the dry film technology is used to build fine line circuits on top of base circuit board made with conventional low density lamination techniques.
In both cases, the package substrate and the larger format circuit board build up are very expensive compared to traditional low density laminate technology, and suffer from several limitations inherent to the process. For example, in the case where a low density laminate base is used as the starting point for subsequent high density layers are built up, the cost increases dramatically since the entire surface of the lower density base board must be processed with the build up process across the entire area, not just in the areas where the high density is required.
Another limitation is the reliability of the via structures joining one circuit layer to another, which tend to be a barrel plated structures with the side walls of the via plated and in many cases must be filled with a via fill material to eliminate an air pocket which may separate during solder reflow temperatures. The vias require drilling through the dry film to expose the previous circuit layer in order to create the via that connects the circuit layers. The dry film is applied as a solid contiguous sheet where the material on that particular layer is restricted to that particular material across the entire layer in the build up less the areas ablated to create the via target for joining the previous and subsequent circuit layers. That is, the dry layer film is homogeneous across the entire layer.